As for the “no system is foolproof”, you’re thinking of implementations, not algorithms. An algorithm can indeed be something-proof. Most “known” algorithms are built on top of very strong mathematical foundations stating what is possible, what is not and what is a maybe.
As for the ads thing, Mozilla is not making a dime off this. It is not monetizable. They’re basically expecting that by giving advertisers a fairly “benign” way to do their shenanigans they will stop doing things the way they currently do (with per-individual tracking).
The absolutists might say that there’s no such thing as benign ads, however truth is that the market forces behind ads are big enough that you’d get website-integrity-bullshit rather ad-free web. Having tracking less ads is better than having a “this website only works in chrome” or “only without extensions” internet.
Is there any other possibility? Maybe. Is is reasonable to think that the moment tracking starts getting blocked em masse, we risk a web-integrity-bullshit +wherever-said-tracking-can-exist-only internet? I think so.